This invention relates to roller mills of the type utilizing a rotating grinding table and one or more grinding rollers adapted to rotate about their own axis and cooperate with the rotating grinding table to grind solid bulk materials such as coal or cement raw meal. More particularly, the present invention relates to an improved apparatus for exerting a downward grinding force on the roller assembly.
Prior to the present invention, roller mills of the type to which the present invention relates are well known. These mills include a grinding table mounted within a closed housing. The grinding table is rotated about a vertical axis by any suitable means such as an electric motor through a gear box. The other grinding element for mills of this type includes a roller assembly mounted within the housing for rotation about its own axis at an angle to the vertical for cooperation with the rotating grinding table to comminute solid material between the rotating grinding table and the roller.
Ground material spills over the edge of the grinding table where it may be picked up by a gas stream flowing upwardly through the housing to a gas solid separator or air classifier which may be located on top of the mill. The classifier rejects oversize particles and returns them for regrinding. Product having a desired fineness will travel out of the classifier to a dust collector entrained in the gas stream with product being separated in the dust collector. It is common practice to utilize the gas for entraining products out of the mill for drying the material.
In order to accomplish grinding on the table, it is necessary to exert a downward grinding force on the roller assembly so that a grinding force is applied to the material between the grinding table and the roller assembly.
Prior to the present invention, it is typical to utilize hydraulic systems to exert a grinding force on the roller assembly. Other systems utilize a combination hydraulic and pneumatic cylinder or mechanical springs to exert the grinding force.
Hydraulic systems and hydrualic pneumatic systems have the disadvantage that the installations can be complex and expensive. Proper pumping, piping and filtration equipment is required in order to maintain the hydraulic system. This is particularly true where equipment is operating in a dusty atmosphere such as is associated with material grinding mills for a cement plant or coal grinding station. In addition, hydraulic systems must be operated at high pressure on the order of 1,000 p.s.i. requiring heavy duty equipment.
A further disadvantage with prior systems is that the hydraulic systems operated through linkages so that all forces are in a downward direction.
A further disadvantage of prior systems for exerting a downward grinding force on roller systems is that they require the use of fabricated steel housings and support systems. This is believed to unnecessarily add to the cost of the roller mill.